The back-end process: Step 4 - Die attach
Today's challenges
BY PETER BÜHLMANN AND DOMENICO TRUNCELLITO

In back-end semiconductor manufacturing, the die attach process is a critical step. In simple terms, die attach is picking a chip from a wafer and placing it onto a substrate or metal lead frame. The way the chip is bonded defines the die attach process - epoxy, soft solder, eutectic and flip chip are the most widely used techniques. Die attach seems to be a simple process step in the semiconductor manufacturing chain. However, the continuously escalating requirements of today's applications create difficult challenges in die bonding.

Typical Die Attach Process Flow

In high-volume production, die attach is performed on fully automated assembly equipment. The basic die attach steps, some of which are performed simultaneously, are:

* A robotic loader picks up a lead frame from a stack and places it on the input area of the workholder.
* The lead frame is moved from the input position to the dispense position. Depending on the required placement accuracy, mechanical or optical alignment points are used to define the dispense position. Epoxy is dispensed in a pattern and volume appropriate for the chip size.
* A sophisticated vision system inspects the lead frame, dispensing pattern and bond pads before the substrate is transported to the bonding position.
* In the meantime, a pattern recognition system locates a good die on the sawn wafer.
* A vacuum pick-up tool mounted on a bond head grabs the aligned die from the wafer and places it on the programmed and pre-dispensed bond position on the substrate.
* Appropriate bonding time and bonding force result in a strong bond, according to the specified process requirements. An additional optical inspection is performed to ensure that placement position and epoxy bleed-out requirements are met.
* Each bond pad on the lead frame or substrate goes through this process before it is unloaded into an output magazine.